1. Field of the Invention
The present invention relates generally to web printing presses and more particularly to a method and device for controlling web tension in a web printing press
2. Background Information
Web printing presses print a continuous web of material, such as paper. During printing, the web of material is acted on by various printing cylinders, which transfer ink to the web. For example, in a four-color double-sided offset lithographic printing press, four pairs of blanket cylinders form nips through which the web passes. During printing, the blanket cylinders contact the web and provide ink to both sides of the web.
During start-up or shut-down, or during plate changes, it is often desirable to have the web press running in a non-printing mode, where the printing cylinders contacting the web are thrown-off so that they do not print on the web. For example, the blanket cylinders in a print unit of a web offset printing press can be separated from each other so that ink is not applied to the web. The web however continues to run through the print unit.
The web in this non-printing condition is known as a white web, since ink is not be applied to the web.
When the press then resumes or begins a printing mode, the tension in the web changes due to the inking and wetting of the web as the web travels through the nips in the printing units. For example, the modulus of elasticity of the web changes due to the application of the ink and any wetting solution. These tension changes in the web can result in web weaving, wrinkling, and increased sensitivity to web defects, often resulting in a web break. Web breaks cause increased press down time created while the web is threaded again through the press. FIG. 1 shows a chart of web tension changes in pounds per linear inch versus time in one example of a prior art web printing press. Tension at an infeed is held constant, for example by a dancer roll and/or PID (proportional, integral, and derivative) controller, as shown by line 1. When impressions are turned on in the press, indicated at about 4 minutes on the time axis, the tension of the web after the printing units drops due to the wetting and inking of the web and the interaction of the printing nip with the web. For example, the tension in the web from the printing units until a first nip in a chill unit is indicated by arrow 2, the tension in the web from the last chill unit nip to a slitter unit is indicated by arrow 3, and a tension in the web from the slitter unit to a folder where the web is cut into signatures is indicated by arrow 4. As can be seen, when impressions are turned on, the tension decreases dramatically for the web as the web travels through the post-printing unit sections.
Active tension controls for a web are known, such as the Bardac SP1070PID Controller for controlling a dancer arm. Tension in the web is adjusted by changing the load on a dancer arm using a PID controller. However, tension controls have not been used to compensate for transient events, such as when impression goes on or off, since active tension control in the post-printing unit sections in response to these events would result in unacceptable variation in the relationship of the image to a cut edge of the resulting signatures.
U.S. Pat. No. 4,838,498 discloses a system for controlling the feeding of web from a supply reel into a web fed printing unit, the system comprising a dancer roll means. U.S. Pat. No. 5,791,541 discloses a tension controller for controlling the tension of running paper web in a rotary press, the tension controller including a floating roller unit. Both of these patents attempt to control the tension caused by speed changes in the infeed, and do not concern themselves with the problems caused by the change in tension in the web due to the wetting and inking of the web. Both of these patents however are hereby incorporated by reference as showing possible dancer roll arrangements for controlling web tension.